Storage material for heat transfer

ABSTRACT

The storage material is intended for the transfer of heat between gas streams in heat exchangers, and has stick-like molded spacing elements (12) on the one hand, and a plurality of plate-like molded elements (12) of heat-resistant plastic serving as storage elements on the other, which are assembled into storage blocks. Polyetherimide is used as the material for the molded elements (10; 12), which combines sufficiently high heat resistance with resistance to aggressive chemical influences.

The invention relates to a storage material for the transfer of heatbetween gas streams in heat exchangers, composed of stick-like profiledspacing elements and a plurality of plate-like molded elements servingas storage elements made of heat-resistant plastic, which are assembledinto storage blocks.

Such a storage material is known from German publication DE-A No. 32 07213. In it, the stick-like profiled spacing elements and theheat-storing, plate-like molded elements are made from polyphenyleneoxide or a copolymer of polyphenylene oxide and polystyrene, or frompolyblends of polyphenylene oxide and polystyrene.

On the other hand, it is the object of the invention further to improvethe long-term stability of the storage material while simultaneouslyachieving a higher temperature level. Moreover, the preparation of thestorage material from the plate-like and stick-like molded elements isto be facilitated.

Setting out from a storage material of the kind mentioned above, thisobject is achieved by making the molded elements from polyetherimide.Imides of high molecular weight are known in the plastics art mainly asheat-resistant thermosets. By the incorporation of imides of highthermal stability and strength into amorphous polyethers by means ofether bonds, i.e., compounds of higher molecular weight having numerousether bonds, polyetherimides are now available which have thermoplasticproperties, i.e., they are easy to form into molded elements on accountof their good rheological qualities and melt stability, and can befabricated by all methods commonly used with thermoplastics. Thus, bothinjection-molding and extrusion methods can be used for making them intostick-like and plate-like molded elements. Due to the high thermalstability, the mass temperatures are to be selected between 340° and425° C., preferably around 400° C. Care must be taken only to see thatheated tools are used for fabrication whose surface has a temperaturebetween 70° and 170° C., preferably around 95° C. By selecting optimumfabricating temperatures, optimum properties are obtained in thefinished molded elements on account of the better rheologicalcharacteristics of the material, such as for example high strength inthe flow seams and high resistance to heat and chemicals, combined withoptimum cycle times despite the amorphous material.

The joining of the stick-like spacing elements and of the plate-likeheat-storing elements can then be performed, as in the known storagematerial, by ultrasonic welding, transitions in the form of studs oftriangular cross section, for example, being provided in the areas to bejoined so as to give a particularly great strength to the bond after theultrasonic welding.

Alternatively, the molded elements can also be joined to one another toform storage blocks by cementing, and this refers both to joining themolded elements by an adhesive cementing using cements on the basis ofpolyurethane, silicone, nonaminic epoxy resins or polyamide hot-meltadhesives, and to joining by means of diffusion cementing using asolvent for polyetherimide, preferably methylene chloride inconcentration of 1% to 5%. In the latter case, the parts to be joinedare softened by the solvent in the area of the junction, and thesesurfaces are bonded together under pressure. After the solventevaporates, a virtually monolithic bonding of the molded elements isachieved, i.e., no foreign matter having material properties differentfrom those of the molded elements is left in the seam. To join bycementing, the areas to be joined are to be fitted accurately to oneanother, and before they are cemented they must be cleansed thoroughlyof grease, oil and dust; isopropyl alcohol is suitable for this purpose.In all cementing processes, furthermore, the application of pressure fora certain length of time after joining is important to the integrity ofthe bond.

Independently of such cementing of the profile elements, or in additionto such cementing, it is desirable, on the basis of the materialproperties as regards their elongation under tensile stress, theirflexibility and their elasticity, to provide the molded elements in theareas of the intended junctions with alternately mating button-like orcleat-like snap-fastening means.

In an alternative embodiment of the invention, the storage blocks can becomposed of perforated plate-like molded elements and stick-like moldedelements provided with rivet heads, the rivet heads being formed orswaged preferably by ultrasonic action.

Particularly when the plate-like molded elements are to have anespecially slight thickness, it may be desirable to reinforce thepolyetherimide used in making them by embedding fibers or fibrousfabrics, preferably glass fibers or glass fiber fabrics, in them. Fibercontents can amount to up to 10%, without thereby losing the resilientproperties of the material which are essential to the creation of snapfastenings.

For assembly into storage blocks by one of the abovementioned cementingmethods, it may be advantageous for the stick-like molded elements to beformed integrally with the plate-like molded elements. Then suchcombination elements can be manufactured by injection molding, or alsoby extrusion, while appropriately configuring the elements used formaking the cementable or snap-fastening junctions. For example, ajunction can be made by providing upstanding portions at the stick-likeareas, these portions having a thickened edge and being snapped intoslots in the plate-like areas of the joining elements.

Since polyetherimide absorbs as much as 0.25% of water within 24 hours,the material should be dried before fabrication to less than 0.05%moisture content, e.g., by heating in a drying oven for a certain lengthof time.

The storage blocks made from the stick-like and plate-like moldedelements or of integral joining elements in accordance with theinvention have a high resistance to aggressive chemicals, such as forexample a large number of hydrocarbon compounds, mineral acids as wellas salt solutions and aqueous lyes with a pH of 9. Furthermore, thestorage blocks have a high stability of shape and a sufficient long-termstability, even over relatively long periods under stress, at higherworking temperatures of up to 170° C.

The invention will be further explained in the description that followsof a number of embodiments, in conjunction with the drawing, wherein:

FIG. 1 is a perspective view of a pack of stick-like and plate-likemolded elements joinable by ultrasonic welding in the preparation of astorage block;

FIG. 2 is a cross sectional view taken through a pack of stick-like andplate-like molded elements configured appropriately for snap-fastening,in the lengthwise direction of the stick-like pieces, as seen in thedirection of the arrows 2--2 in FIG. 3;

FIG. 3 is a cross-sectional view taken along the arrows 3--3 in FIG. 2;

FIG. 4 shows a pack of molded elements produced by snap-fastening, withmolded elements different from those used in FIGS. 2 and 3, in across-section similar to FIG. 2;

FIG. 5 is a cross-sectional view taken in the same manner as FIGS. 2 and4 through a pack of stick-like and plate-like molded elements configuredin a manner suitable for joining by cementing;

FIG. 6 is a cross-sectional view seen in the direction of the arrows6--6 in FIG. 5;

FIG. 7 is a cross-sectional view taken in the same manner as FIG. 6through a pack of molded elements in which the stick-like moldedelements are joined to the plate-like molded elements by riveting withrivet heads produced by ultrasonic forming; and

FIG. 8 is a cross-sectional view taken in the same manner as FIG. 6 or 7through a pack of molded elements having cleat-like molded elementsformed integrally on the plate-like molded elements.

In FIG. 1, seven plate-like molded elements 10 alternate with stick-likespacing moldings 12 in three rows one over the other, all joinedtogether by ultrasonic welding. The next-following layer of stick-likespacing moldings 12 has already been laid on the top molded element 10of the plate pack. For welding to the next (not shown) molded element,the molded spacing elements 12 are provided on both sides, i.e., also onthe bottom not visible in the drawing, with the cleats 14 of triangularcross section disposed in the axial direction and represented on theirupper side, on the one hand, and on the other hand they are providedwith circular studs 16 for spot welds. Prior to the ultrasonic welding,first the next plate-like molded element is placed on the uppermostlayer of the stick-like spacing element 12. The ultrasonic welding thustakes place step-wise simultaneously through a remote weld joining thebottom of the stick-like molded elements 12 to the plate-like moldedelement beneath them, and through a proximate weld to join their upperside to the plate-like molded element that is yet to be placed on them.

The stick-like molded spacing elements are provided, in the area of thespot welds to be made on them, with rounded, circular-shaped expansions,so that in these areas particularly sturdy, button-like junction areasare formed. In the section of the plate pack represented in the drawing,the rectilinear front edges of the plate-like molded elementsrepresented on the right side of the drawing are situated at the area ofthe later entrance of the dust-laden gases, whose direction of flow maybe symbolized by the arrow G. The cleaning jets of a soot blower, whosedirection of action may be represented by the arrow B, act in the samedirection.

The molded spacing bars 12 are provided with expansions which are setback from the above-mentioned entry front and have the studs 16, ofcircular plan, for the ultrasonic spot welds.

Immediately adjacent the entrance end there are provided the prismaticprojections 18, shorter than the other prismatic projections 14 runninglengthwise of the stick-like moldings, which serve only for aid inassembly. They fix the storage block or pack of molded elements in thearea of the entrance end, where they prevent deformation by theultrasonic welding performed stepwise during the assembly of the storageblock from the plate-like and stick-like molded elements. In lateroperation, the welds made with the projections 18 can break open, sothat the plate-like molded elements 10 will form a section capable ofvibration in the entrance area.

In the case of the pack shown in FIGS. 2 and 3, the the plate-likemolded elements are snap-fastened to the stick-like molding elements 12;for this purpose, studs 20 whose heads are thickened project from theopposite flat sides of the stick-like molded elements 12 at regularintervals, and the studs are snapped into associated bores 22 in themolded elements 10. The studs 20 are opposite one another on the top andbottom of the stick-like molded elements 12, as it can be seen in FIG.2. On the other hand, the stick-like molded elements 12 are spaced apartbetween successive plate-like molded elements 10 by the amount ofspacing provided between adjacent bores 22 in the molded elements 10.

In the pack of molded elements shown in FIG. 4, the stick-like moldedelements 12 again have studs 20 with thickened heads, which are snappedinto associated bores 22 in the plate-like molded elements 10; the studs20 on opposite sides of the stick-like molded elements, however, arespaced apart by an amount which is equal to twice the distance betweencenters of adjacent bores 22 in the plate-like molded elements, and thestuds 20 on the top and bottom of the molded elements 12 are offset fromone another by the distance between centers of adjacent bores 22 in themolded elements 10. It is thus possible to arrange the stick-like moldedelements 12 in packs one over the other without lateral misalignmentbetween successive molded elements 10, as in the case of the plate packrepresented in FIG. 1.

FIGS. 5 and 6 show a pack of molded elements constructed similarly tothe pack shown in FIGS. 2 and 3; in this pack, however, the moldedelements 10 and 12 are not snap-fastened together but joined bycementing. Instead of the studs 20 of circular cross section, elongatedprojections 24 are formed on the stick-like molded elements, and theseprojections fit matingly into associated slots 26. To join the moldedelements 10 and 12 together, the projections are inserted into the slots26 with a cement, or the surfaces which engage one another in the slotsare softened with a solvent prior to the assembly of each molded element10. After the solvent evaporates, the projections 24 are then bonded ina virtually monolithic manner to the associated walls of the slot 26.

In FIG. 7 is shown a method of joining the stick-like molded element 12to the plate-like molded elements 10, in which, again, studs of circularcross section projecting from the opposite flat sides of the stick-likemolded elements 12 are inserted into associated bores 30 in theplate-like molded elements 10. The fixing of the studs 28 in the bores,however, is accomplished by forming rivet heads 32 on the free ends ofthe studs by swaging these stud ends with ultrasound.

Lastly, FIG. 8 shows still another pack of combination elements 32, inwhich the stick-like molded elements are formed virtually integrallywith the plate-like molded elements 10. In the present case, differentstick-like molded elements 34 and 36, alternating and set apart from oneanother each by the spacing between adjacent stick-like molded elementsare provided on opposite sides of the plate-like molded elements 10. Theone molded element 34 of these elements is a relatively narrow stripprovided on its free end with a head portion of circularly thickenedcross section, while the second molded element 36 is in the form of awider stip having a longitudinal groove complementary to the moldedelement 34. The molded elements 34 of the one plate 32 can thus besnapped into the molded elements 36 of the next plate 32 in the pack.

What is claimed is:
 1. Storage material for the transfer of heat betweengas streams in heat exchangers, composed of stick-like molded spacingelements and a plurality of plate-like molded elements (12) serving asstorage elements of heat-resistant plastic, which are assembled intostorage blocks, characterized in that the molded elements (10; 12; 32)are made from polyetherimide.
 2. Storage material of claim 1,characterized in that the stick-like molded spacing elements and theheat-storing plate-like molded elements (12; 10) are joined together byultrasonic welding.
 3. Storage material of claim 1, characterized inthat the molded elements (10; 12; 32) are assembled together intostorage blocks by cementing.
 4. Storage material of claim 3,characterized by a bonding of the molded elements (10; 12; 32) by meansof adhesive cementing with the aid of cements on the basis ofpolyurethane, silicone, nonaminic epoxy resins or polyamide hot-meltadhesives.
 5. Storage material of claim 3, characterized by a bonding ofthe molded elements (10; 12; 32) by means of diffusion cementing using asolvent for polyetherimide, preferably methylene chloride, in aconcentration of 1 to 5%.
 6. Storage material of claim 1, characterizedin that the plate-like and the stick-like molded elements (10; 12; 32)have button-like or stick-like joining member (20; 24; 26) constitutingsnap fasteners in the junction areas to serve as alternating male andfemale fasteners for assembling the storage block.
 7. Storage materialof claim 3, characterized in that the plate-like molded elements (10)with stick-like molded elements (34; 36) formed thereon are constructedas integral combination elements (32).
 8. Storage material of claim 1,characterized in that the storage blocks are composed of perforated,plate-like molded elements (10) and stick-like molded elements (12)provided with rivet heads.
 9. Storage material of claim 8, characterizedin that the stick-like molded elements (12) are joined with theplate-like molded elements (10) to form the storage block by swaging therivet heads by means of ultrasound.
 10. Storage material of claim 1,characterized in that the storage blocks are composed of slotted,plate-like and stick-like molded elements (10; 12), the stick-likemolded elements (12) having molded studs (24) which are snapped into theslots (26) in the plate-like molded elements (10).
 11. Storage materialof any of claim 1, characterized in that the polyetherimide isreinforced by embedded fibers or fiber fabrics, preferably glass fibersor glass fiber fabrics.